Chuck is correct that they're not typically recommended for autos, but specifically .30-06 Lt. mags aren't supposed to be fired in M-1Garands, and if memory serves .308 Lt. mags aren't to be used in M-1A's, because of the nature of their gas system, it's a bit more sensitive to pressure variances than other known autos, and it can be damaged by high pressure rounds.

They're pretty much a waste of money, from my experience. As much as they cost you'd think they'd make a .30-06 good for deer to 2000yrds or elephant at 500yrds, while they pretty much don't do anything but give you a little more umph, and not really enough umph that they're worth shooting or buying. They tend to add a good bit of recoil too, so you've got a harder kicking gun that's more expensive to shoot, but that isn't any more accurate at any longer range nor is it really capable of any larger game species than with standard loads.

Basically, they're a marketing gimic.